Satellites provide a vital communication link for radio, telephone and television traffic in addition to other forms of data communication. Satellite communication systems are useful alternatives to conventional terrestrial communication systems, such as land lines, fiber optic, microwave, and the like.
One example of satellite communication, well-known in the industry, employs communication satellites in geosynchronous orbit around the Earth. Such geosynchronous orbits require the insertion of satellites at a location approximately 22,300 miles from Earth near the equator. In this location, a satellite orbits the Earth at a velocity that matches the Earth's rotational velocity. Thus, a geosynchronous satellite remains in a substantially fixed position relative to the Earth's surface.
Geosynchronous satellites have the advantage of being in a fixed position relative to the surface of the Earth. However, geosynchronous satellites are extremely expensive due to the high cost of insertion into a geosynchronous orbit. Furthermore, Earth-based stations communicating with a geosynchronous satellite require high power transmission levels to effectively communicate with a satellite 22,300 miles away. In addition, the satellite itself must have a high power transmitter to effectively communicate with Earth-based receivers. Transmission delays, due to the time required for radio signals to propagate up to a satellite and back to Earth, are also a significant problem with geosynchronous satellite systems.
A significant advantage of geosynchronous satellites is the ability to position satellites far enough from each other in a geosynchronous orbit so as to permit Earth-based antenna systems to discriminate between the various satellites. For example, an antenna on an Earth-based station can be positioned so as to communicate with a particular geosynchronous satellite while minimizing interference to and from adjacent satellites in geosynchronous orbit. This is especially important when geosynchronous satellites share a common portion of the radio frequency spectrum.
To overcome the disadvantages of transmission delay associated with communications using a geosynchronous satellite communication system, a series of low-Earth orbit (LEO) satellites may be inserted into non-geostationary orbits. Such LEO satellites suffer from the disadvantage that they are not in a fixed location with respect to the Earth's surface. However, LEO satellites offer the advantage of low transmission delay. Satellite systems, whether using geosynchronous satellites or LEO satellites, must effectively use the radio frequency spectrum assigned to the communication system.
With a low-Earth orbit, the satellites move relative to the Earth, and relative to each other. This creates potential radio frequency interference problems when additional communications systems that are also in low-Earth orbit are expected to share the same radio frequency spectrum. Therefore, it can be appreciated that there is a significant need for a system and method for sharing radio frequency spectrum among LEO satellite communication systems. The present invention provides this and other advantages as will be apparent from the following description and accompanying figures.